1. Field of the Invention
The present invention relates, generally to an automatic transmission for a vehicle and, more specifically, to an automatic transmission having a start-up clutch.
2. Description of the Related Art
Conventional multi-stage or multi-gear automatic transmissions are generally provided with a start-up device for providing the necessary torque translation from the engine to the transmission to initiate movement of the vehicle from a stationary position. Most often, the start-up device takes the form of a fluid-coupled, or hydrodynamic torque converter. The torque converter provides torque amplification at vehicle start-up through a predetermined vehicle speed at which point the fluid coupling is no longer needed.
An example of a conventional automatic transmission having a hydrodynamic torque converter as start-up device is schematically illustrated in FIG. 4. The automatic transmission 26 includes a transmission housing 7 having an input shaft 1 and an output shaft 2. While various component arrangements have been employed to effect gear changes to establish various gear ratio translation relationships between the input and output shafts 1 and 2, this example includes an input planetary gear assembly 4, a double output planetary gear assembly 5, three clutches A, B, and E, and two brakes C and D.
The input planetary gear assembly 4 includes a sun gear 12, a planetary gear 13 held by a planet gear carrier, or planetary arm 15, and an internal, or ring gear 14. As illustrated, the sun gear 12, the planetary gear 13, and the ring gear 14 engage in a conventional manner, namely the sun gear 12 is in engagement with the planetary gear 13, and the planetary gear 13 with the ring gear 14.
The double output planetary gear assembly 5 likewise includes a first sun gear 16, a first planetary gear 17 supported by a first planet gear carrier, or planetary arm 21, a second sun gear 20, a second planetary gear 18 held by a second planet gear carrier, or planetary arm 22, and a ring gear 19. The first sun gear 16 and the first planetary gear 17 both operatively engage the second planetary gear 18 of the output planetary gear 5.
A further sun gear 20 is shown in this illustrative embodiment. As shown in FIG. 4, the input shaft 1 is connected with the ring gear 14 of the input planetary gear assembly 4. The sun gear 12 of the input planetary gear assembly 4 is fixed against rotation to the transmission housing 7. The planet arm 15 is selectively connectable to the first sun gear 16 of the output planetary gear assembly 5 via the clutch A and is also selectively connectable with the second sun gear 20 via clutch B.
The two joined planet arms 21 and 22 of the output planetary gear assembly 5 are selectively connectable to be fixed against rotation with the gear housing 7 via the brake D. Clutch E provides a selective connection between the joined planet arms 21 and 22 and the input shaft 1. The second sun gear 20 is either selectively connectable to the planet arm 15 of the input planetary gear assembly 4 via the clutch B, or is selectively connectable as a connection fixed against rotation to the gear housing 7 by brake C. Additionally, the ring gear 19 is directly connected to the output shaft 2.
Thus, the selective engagement of the five shift elements, namely the three clutches A, B, and E as well as the two brakes C and D effects the various gear translation relationships, or ratios between the input shaft 1 and the output shaft 2 by controlling which gear elements of the planetary gear assemblies 4 and 5 operatively effect the rotation of the others. In this example of the prior art, there are six forward gears, one neutral position, and one reverse gear. As further illustrated in FIG. 4, this automatic transmission 26 employs a hydrodynamic torque converter 3 as a start-up device. The torque converter 3 includes an impeller 8, a turbine wheel 9 and stator 10. The turbine wheel 9 is fixedly mounted to the input shaft 1 and the stator 10 is connected with the gear housing 7 via a one-way clutch 11.
The impeller 8, turbine wheel 9 and stator 10 further include a plurality of blades or vanes that act to direct the movement of hydraulic fluid so that the mechanical energy of the rotating impeller 8 is converted to hydrokinetic energy, transferred to the turbine wheel 9 through the stator 10 and then converted back into mechanical energy to rotate the turbine wheel 9 and the input shaft 1. The torque converter 3 further includes a lock-up clutch 6 that makes a direct connection between the motor shaft M and the input shaft 1 after the vehicle has reached a predetermined speed where the fluid coupling of the torque converter 3 is no longer needed.
Although this type of arrangement has generally worked well for its intended purpose, there remains room for improvement. More specifically, the torque converter is large and bulky and requires a pump (not shown) to supply hydraulic fluid to the impeller. The size, weight and fluid coupling of the torque converter cause parasitic losses that reduce the efficiency of the transmission and the vehicle. Further, the torque converter and its supporting elements are complex and costly to produce.
Accordingly, there remains a need in the related art for an automatic transmission having a start-up device that eliminates the need for a hydrodynamic torque converter, has less size and weight, and provides greater efficiency and more cost effective production by employing fewer components. There also remains a need for this type of transmission that also continues to provide a smooth automatic transition for the vehicle from stationary to moving.